The technical field relates to a process for the fabrication of a porous coating on top of an open-structure substrate. The porously coated open-structure substrate is preferably used in fuel cell applications.
A fuel cell is an electrochemical apparatus wherein chemical energy generated from a combination of a fuel with an oxidant is converted to electrical energy in the presence of a catalyst. The fuel is fed to an anode, which has a negative polarity, and the oxidant is fed to a cathode, which, conversely, has a positive polarity. The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode.
One of the essential requirements of typical fuel cells is the easy access to the catalyst and a large surface area for reaction. This requirement can be satisfied by using an electrode made of an electrically conductive porous substrate that renders the electrode permeable to fluid reactants and products in the fuel cell. To increase the surface area for reaction, the catalyst can also be filled into or deposited onto a porous substrate.
However, these modifications result in a fragile porous electrode that needs additional mechanical support. An alternative is to sinter a porous coating on a solid substrate and then fill or re-coat the porous coating with a catalyst. The substrate can be made of conductive materials or patterned with a conductive material. The porous coating can be made of non-conductive materials, such as ceramics or silicon, or conductive materials such as carbon, ceramic-metal mixture or metals. Typically, the coating material, in the form of fine powders, is mixed with a liquid organic xe2x80x9cbinderxe2x80x9d such as glycol to form a coating mixture. The coating mixture is spread on the substrate and is baked in an oven. The binder is burned off and the coating material is sintered on the substrate to form a porous layer. A catalyst is deposited in the porous layer to provide a large catalytic surface. The substrate is then etched from the backside to create openings so that fuel on the cathode side and oxygen on the anode side can reach the active catalytic surfaces through the openings and the porous layer. The etching process, however, is time consuming and requires specially designed machinery.
A method for sintering an open-structure substrate with a porous coating is disclosed. The open-structure substrate is a substrate with pre-formed openings (i.e., pre-formed pores, channels, passageways, etc.), which allow liquids and gases to pass from one side of the substrate to the other side of the substrate. The method is based on the fact that a viscous solution can remain a continuous layer after being applied onto open-structure substrates, such as screens or expanded foils, without dripping through the openings of the substrate.
Briefly, for a chosen open-structure substrate, a coating paste is prepared by mixing a solid coating material with a liquid binder. The coating material and the binder is mixed at a ratio such that the viscosity of the paste is high enough to prevent the paste from dripping through the openings on the substrate. The paste is spread on the surface of the open-structure substrate, and is subjected to a heating process to remove the binder and to sinter the coating material on the surface of the open-structure substrate to form a porous coating. Since the openings on the substrate are pre-formed before the coating process, this method eliminates the expensive and time-consuming etching step after the sintering. The method can be used to manufacture electrodes for fuel cells or any other applications which require a porous coating on an open-structure substrate.
In an embodiment, the paste comprises fine metal powders, such as zinc or silver powders, a viscous binder, such as glycol, and, optionally, a flux. A flux is a reducing agent that serves to remove the oxidized surface layer of metal particles.
In another embodiment, the porous coating on the open-structure substrate may be further coated with a catalyst.
In yet another embodiment, the paste may be prepared in the form of a solgel. A solgel is prepared by peptizing a coating material, such as silicon oxide or metal oxide, with water or a water-miscible alcohol, such as methanol, ethanol, isopropanol, ethylene glycol and the like, to form a viscous polymeric sol. The viscous polymeric sol is heated at a relatively low temperature (usually less than 100xc2x0 C.) to form a heat-set gel. The heat-set gel is then heated in the presence of oxygen at a temperature and for a period of time sufficient to oxidize and volatilize any remaining vapors and organic materials from the gel to form a solid porous product.